1. Field of the Invention
This invention relates to the field of consolidating bodies, and more specifically, to an improved method which enables metallic bodies to be made with minimal distortion.
2. Prior Art
Methodology associated with producing high density metallic objects by consolidation is recognized in the prior art. Exemplars of prior art references which discuss such methodology are U.S. Pat. Nos. 3,356,496 and 3,689,259. Prior to discussing these references, a brief discussion will be set forth which illustrates the two primary methodologies currently used to densify either loose powder or a prepressed metal powder compact. These two techniques are generally referred to as Hot Isostatic Pressing and Powder Forging. The Hot Isostatic Pressing ("HIP") process comprises placing loose metal powder or a prepressed compact into a metal can or mold and subsequently evacuating the atmosphere from the can, sealing the can to prevent any gases from reentering, and placing the can in a suitable pressure vessel. The vessel has internal heating elements to raise the temperature of the powder material to a suitable consolidation temperature. Internal temperatures of 1000.degree. C. to 2100.degree. C. are typically used depending upon the material being processed. Coincident with the increase in the internal temperature of the HIP vessel, the internal pressure is slowly increased and maintained at from 15,000 to about 30,000 psi again depending upon the material being processed. Under the combined effects of temperature and isostatic pressure, the powder is densified to the theoretical bulk density of the material.
A HIP vessel can accept more than one can during a given cycle and thus there is the ability to densify multiple powdered metal articles per cycle. In addition, by the use of isostatic pressure, the densification is more or less uniform throughout the HIPed article. By the use of suitable can design, it is possible to form undercuts for transverse holes or slots in the densified article. However, the cycle time of the charge is slow, often requiring 8 hours or longer for a single cycle. Further, at the completion of the cycle, the cans surrounding the powdered metal article have to be either machined off or chemically removed.
The second common method of densifying powdered metal is a technique referred to as Powder Forging ("PF"). The Powder Forging process comprises the steps of:
(a) cold compacting loose metal powder at room temperature in a closed die at pressures in the range of 10-50 TSI into a suitable geometry (often referred to as a "preform") for subsequent forging. At this stage, the preform is friable and may contain 20-30 percent porosity and its strength is derived from the mechanical interlocking of the powdered particles. PA1 (b) sintering the preform (i.e. subjecting the preform to an elevated temperature at atmospheric pressure) under a protective atmosphere. Sintering causes solid state "welding" of the mechanically interlocked powdered particles. PA1 (c) reheating the preform to a suitable forging temperature (depending upon the alloy). Alternately this reheating step may be incorporated into the sintering step. PA1 (d) forging the preform in a closed die into the final shape. The die is typically maintained at a temperature of about 300.degree. F. to 600.degree. F.
The forging step eliminates the porosity inherent from the preforming and gives the final shape to the PF part.
Advantages of Powder Forging include: speed of operation (up to 1000 pieces per hour), ability to produce a net shape, mechanical properties substantially equivalent to conventionally forged products and increased material utilization. However, there are number of disadvantages including nonuniformity of density because of chilling of the preform when in contact with the relatively cold die, and the inability to form undercuts which can be done in HIP.
Now referring back to the patents mentioned above, such references disclose what appears to be a combination of isothermal and isostatic conditions of HIP and HIP's ability to form undercuts, with the high speed, low cost continuous production normally associated with Powder Forging. In the '496 patent, the use of a cast ceramic outer container is taught as the primary heat barrier. In addition, this cast ceramic outer container when deformed causes nearly uniform distribution of pressure on the powdered material.
In the '259 patent the use of granular refractory materials is taught. This reference is intended as an improvement over the earlier '496 patent in relation to faster heating of the grain and faster heating of the prepressed part.
While the '496 and '259 patents may represent advances in the art, significant problems remain with respect to the use of a bed of ceramic into which a preform is placed prior to consolidation. More specifically it has been found that the use of crushed and ground ceramics or carbides results in a significantly non-uniform pressure distribution from the top of the charge (the surface against the moving press member) to the bottom of the charge (the surface against the fixed press bed). This non-uniformity of pressure distribution is readily demonstrated when consolidating a prepressed right circular cylinder of a powdered material. After consolidation in a bed of crushed and ground or fused ceramic material to nearly 100% of bulk density, it was determined that the surface of the prepressed cylinder nearest the moving press ram was smaller in diameter than the surface nearest the fixed bed. Sectioning the consolidated cylinder along a diameter and examining the sectioned surface, indicated that it had the shape of a trapezoid. The above phenomena was observed in all consolidated articles when a crushed and ground or fused granular ceramic matrix was employed as the consolidation media.
The solution to the problems associated with such distortion and lack of dimentional stability in shape has proved ellusive, especially when the solution must also be applicable to mass production. It has recently been determined that the use of generally spheroidal ceramics particles, especially when coated with a thermally stable lubricant, overcame most of the distortion problems. However, the use of a ceramic bed will inherently lead to embedding of the ceramic particles into the surface of the preform. This creates surface imperfections which can adversely affect strength, functionality and aesthetic appearance. The present invention provides a solution to this problem.